Dating from prehistoric times, diamond has had a mythical importance to many civilizations and, as a result, has one of the highest values of all naturally occurring materials. Diamond is, of course, a crystalline form of carbon whose beauty and value derive from its extraordinary hardness, toughness and high refractive index. It is primarily the refractive index that gives diamond its characteristic brilliance.
Simulated diamond materials have entered the marketplace with varying degrees of success. The most widely distributed simulated diamond material is cubic zirconia. While cubic zirconia usually can be distinguished from diamond through visual inspection by a qualified jeweler, a test comparing physical properties of cubic zirconia versus diamond is desirable to prevent improper identification of a gemstone. In this regard, cubic zirconia has a lower refractive index than diamond. Also, it is considerably softer, has a much higher density and has a significantly lower thermal conductivity. One relatively straightforward and reliable nondestructive test for distinguishing cubic zirconia from natural diamond is set forth in U.S. Pat. No. 4,255,962 which describes an apparatus including a thermal probe held in physical contact with the gemstone being tested, and associated circuitry for determining the thermal conductivity of the gemstone.
Recently, it has been discovered that single crystals of synthetic silicon carbide can be grown with desired color characteristics and thereafter faceted and polished into synthetic gemstones. These gemstones have extraordinary hardness, toughness, chemical and thermal stability, and a high refractive index that produces unparalleled brilliance.
Synthetic silicon carbide crystals can be grown in a wide range of colors and shades within each color by the appropriate selection of dopants and by varying the net doping densities. The silicon carbide crystals can also be grown colorless. Thus, silicon carbide crystals offer the potential to be faceted and polished into gemstones of many various appearances, including that of diamond.
A comparison of certain important physical properties of diamond, silicon carbide and cubic zirconia is as follows:
______________________________________ Thermal Mohs Refractive Density Conductivity Hardness Index (SG) (W/cm .multidot. K) ______________________________________ Natural Diamond 10 2.42 3.5 6.6 Silicon Carbide 9-9.25 2.69 3.2 4.9 (6H polytype) Silicon Carbide 9-9.25 2.71 3.2 4.9 (4H polytype) Cubic Zirconia 7.75-8.5 1.98 5.8 0.02 ______________________________________
Even for an experienced jeweler or diamond merchant, the difference between diamond and a colorless silicon carbide gemstone is difficult, if not impossible, to see with the naked eye. The skilled use of the usual optical tools employed in the industry still do not produce reliable results in distinguishing between the two materials. In fact, certified gemstone appraisers are known to have mistakenly identified silicon carbide as diamond. The differences that may appear in color are of no significance since silicon carbide gemstones may be produced in colorless form or with a slight tint. For example, a silicon carbide gemstone may be produced with the light shading of blue that is found in certain diamonds, including some of the rarest and most expensive diamonds, such as the Hope Diamond. As shown in the above table, the differences in hardness, refractive index, density and thermal conductivity between diamond and silicon carbide are not sufficiently great to form a basis for a reliable, easy to use testing procedure and apparatus that can be employed by typical jewelry stores and appraisers.
Other sophisticated laboratory tests might be considered for application to the problem of distinguishing silicon carbide gemstones from natural diamond gemstones. However, even many complicated and otherwise reliable techniques are limited in their application to this problem because silicon carbide can form in more than 150 different atomic arrangements (polytypes) each having different physical and electronic characteristics. In addition, the hexagonal polytypes of silicon carbide have properties that are different in each crystallographic plane. For instance, ultraviolet fluorescence of silicon carbide has little value because many silicon carbide polytypes (including 6H) do not fluoresce. Since a certain percentage of natural diamond also does not fluoresce, ultraviolet fluorescence cannot reliably distinguish between silicon carbide and diamond. Furthermore, spectrometry and x-ray techniques are not necessarily appropriate to the task because they are highly dependent on the skill of the operator and are far too complex and/or time-consuming and/or expensive to be employed on a routine basis in the tens of thousands of businesses that must now be concerned with distinguishing between diamond and silicon carbide gemstones.
With the advent of synthetic silicon carbide gemstones, and particularly colorless and lightly tinted silicon carbide gemstones, there has developed an acute need for a reliable and cost-effective procedure and apparatus for use by jewelry stores, appraisers and pawn shops to positively identify silicon carbide gemstones and thereby distinguish them from diamond, cubic zirconia and other gemstones, with the end goal of preventing intentional or unintentional misindentification of these gemstones.